Field of the Invention
The invention relates to a digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is intended for use in the 2.4 GHz ISM (Industrial Scientific and Medical) frequency band, in which a 20 dB channel bandwidth of a maximum of 1 MHz is prescribed and rust be adhered to.
The ISM frequency band from 2400 MHz to 2485 MHz is available worldwide for industrial, scientific and medical purposes. This UHF frequency band car be used to provide radio services, that is to say including cordless telephones. However, in particular, exactly prescribed conditions must be adhered to for radio operation in this frequency band.
If a digital multi-subscriber radio telecommunication system operating using frequency hopping technology is to be produced in this 2.4 GHz ISM band, then it is a significant feature that a 20 dB bandwidth of a maximum of 1 MHz is prescribed for this purpose and must be strictly adhered to; this is known to be exceeded in the case of the DECT standard.
A digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is intended for use in the 2.4 GHz ISM (Industrial Scientific and Medical) frequency band is nor known to date. For the WLAN standard IEEE 802.11, a data transfer rate of 1 Mbit/s is stipulated for GFSK (Gaussian Frequency Shift Keying) modulation. However, this results in markedly higher demands on accuracy for the modulation deviation in the radio-frequency part of the radio sets and hence in a much higher technical and hence cost outlay than in DECT units, for example.
The object of the invention is to provide, for the 2.4 GHz ISM frequency band, a digital multi-subscriber radio telecommunication system which operates using frequency hopping technology and is based on an already widespread radio system of this type, so that only relatively small alterations are required in the units used for this system when used as desired in the ISM band, and the extent of the alterations in the digital modules is so small that said modules can be incorporated in the units of the radio system which is already used without any significant additional technical and hence cost outlay.
In accordance with the invention, which is based on a digital multi-subscriber radio telecommunication system of the type mentioned in the introduction, this object is achieved by a modification to the known DECT (Digital European Cordless Telecommunications) radio system, which operates using TCM. (Time Division Multiple Access)/FDMA (Frequency Division Multiple Access) technology, such that the data transfer rate is reduced by comparison with the data transfer rate of 1152 kbit/s which is prescribed as standard for DECT, and that there is provision for the radio-frequency parts of the units used in the system to be matched to the ISM frequency band, which is altered by comparison with the DECT standard.
The invention thus provides a frequency hopping system in the 2.4 GHz ISM band. It has been recognized that the DECT system is frame duration of 10 ms means that the system is capable of making the required frequency changes, where a frequency should He maintained for a maximum of 0.4 s in 30 s.
Expedient embodiments, developments and one possible se for the system according to the invention are specified in the dependent claims.
According to one advantageous embodiment of the invention, the data transfer rate is reduced to half the data transfer rate prescribed in the DUCT standard, that is to say is reduced from 1152 kbit/s to 576 kbit/s. In this case, two time slots are expediently combined in order to maintain the net data transfer rate. The channel spacing is expediently likewise reduced to half the DECT channel spacing, that is to say is reduced from 1532 kHz to 864 kHz.
The Frequencies can thus be readily generated from a standard DECT system clock. In the radio-frequency part of the units used, only one new IF filter with half the bandwidth is required in addition to the front end""s being matched to the new frequency range.
In principle, a reduction to another fraction of the DECT data transfer rate, e.g. to ⅔ or xc2xe of the DECT data transfer rave, with another channel spacing is also possible. However, this may result in additional outlay by comparison with the reduction to half the DECT data transfer rate and to half the DECT channel spacing.
An illustrative embodiment of a subscriber unit for the digital multi-subscriber radio telecommunication system for use in the 2.4 GHz ISM frequency band is explained below and is shown in the drawings.